Quantification of perceptual sensitivity to latency in virtual environments (VEs) and elucidation of the mechanism by which latency is perceived is essential for development of countermeasures by VE designers. We test the hypothesis that observers use "image slip" @e., motion of the VE scene caused by system time lags) to detect the consequences of latency rather than explicitly detecting time delay. Our presumption is that forcing observers to change from constant rate to randomly paced head motion will disrupt their ability to discriminate latency based on perceived image slip. This study indicates that the disruption in motion pattern causes a shift in latency detection criteria and a minor degradation in discrimination ability. It is likely therefore that observers make at least some use of image slip in discriminating VE latency. It can also be inferred that when observers learn to discriminate latency, their Just Noticeable Difference (JND) remains below 17 ms.

System latency (time delay) and its visible consequences are fundamental virtual environment (VE) deficiencies that can hamper user perception and performance. The aim of this research is to quantify the role of VE scene content and resultant relative object motion on perceptual sensitivity to VE latency. Latency detection was examined by presenting observers in a head-tracked, stereoscopic head mounted display with environments having differing levels of complexity ranging from simple geometrical objects to a radiosity-rendered scene of two interconnected rooms. Latency discrimination was compared with results from a previous study in which only simple geometrical objects, without radiosity rendering or a 'real-world' setting, were used. From the results of these two studies, it can be inferred that the Just Noticeable Difference (JND) for latency discrimination by trained observers averages ~15 ms or less, independent of scene complexity and real-world meaning. Such knowledge will help elucidate latency perception mechanisms and, in turn, guide VE designers in the development of latency countermeasures.

A head-mounted visual display was used in a see-through format to present computer generated, space-stabilized, nearby wire-like virtual objects to 14 subjects. The visual requirements of their experimental tasks were similar to those needed for visually-guided manual assembly of aircraft wire harnesses. In the first experiment subjects visually traced wire paths with a head-referenced cursor, subjectively rated aspects of viewing, and had their vision tested before and after monocular, biocular, or stereo viewing. Only the viewing dificulty with the biocular display was adversely effected by the visual task. This viewing dtfficulty is likely due to conflict between looming and stereo disparity cues. A second experiment examined the precision with which operators could manually move ringshaped virtual objects over virtual paths without collision. Accuracy of per$ormance was studied as a function of required precision, path complexity, and system response latency. Results show that high precision tracing is most sensitive to increasing latency. Ring placement with less than 1.8 cm precision will require system latency less than 50 msec before asymptotic per$ormance is found.Introduction.

User perceptual sensitivity to changes of system latency was tested in three simple virtual environments: one with only a foreground object, a second with only a background object, and a third that combined both of these elements. Prior psychophysical measurements of sensitivity, Just Noticeable Difference; and bias, Points of Subjective Equality, from our laboratory are confirmed with measurements in 13 subjects. Our measurements indicate that perceptual stability across a variety of virtual environments will require latencies less than 16 ms. We discount a possible explanation that the differences between our results and those from a study by Allison et al. could be related to a visual capture effect initially reported by L. Matin. Instead, the differences may be due to the type of psychophysical judgment rendered by the subjects and the degree to which subjects were instructed and practiced.